1. Field of Invention
The present invention relates in general to cardiac surgery, and, more specifically, to the heating and cooling of blood or other fluids delivered to a patient during cardiac bypass surgery.
2. Background of Related Art
Heating and cooling devices are an important part of blood perfusion systems used during cardiac surgery. During surgery, blood is cooled in a bypass circuit to induce hypothermia to protect the organs. When the surgery has been completed, the blood is re-warmed prior to the patient waking from anesthesia. A “cooler heater” unit device in the bypass circuit is used to cool and heat the cardiac fluid.
A heat transfer fluid such as purified water circulates through a pump in a first circuit and is selectably heated or cooled before being sent to a heat exchange unit. A second circuit carries cardiac fluid to the heat exchange unit. The heat exchange unit includes two chambers. The chambers are separated by a thermally conductive barrier. The warm/cold water (or other heat exchange fluid) in the first circuit is circulated through a first chamber of the heat exchange unit. The cardiac fluid from the patient is circulated through a second chamber of the heat exchange unit. The warm/cold water and the cardiac fluid are each contained in their respective closed circuits, and as a result, are not allowed to mix. The warm or cold water circulated to the first chamber of the heat exchange unit either adds or removes heat as necessary from the cardiac fluid circulated through the second chamber.
The system for providing the temperature controlled water to the heat exchange unit typically includes a cold storage tank, a manifold (i.e., heater and temperature sensing unit), a pump, valves, and water ports. The cold storage tank allows for a volume of water to fill the first circuit, the water side of the heat exchanger, and the portion of the tank contacting the cooling media (e.g., ice). The manifold typically contains a heating element and temperature sensing devices for determining the current temperature of the water flowing to the heat exchange unit. The valves include a set of isolation valves which are actuated between an open or closed position to determine whether water is circulated to the cold storage tank. An electronic controller controls the valve positions and selectably activates a heating device when heating is required. The combined actuation of the valves in cooperation with the heating element regulates the temperature of the water provided to the heat exchange device.
After a temperature setting is input to the controller, the controller measures the temperature of the water flowing to the heat exchange unit and determines whether the temperature needs to be increased, maintained or decreased. If the temperature needs to be increased, then a solenoid valve supplying the cold water from the tank is closed and the heater is turned on. If the temperature needs to be decreased, then the heater is turned off and the solenoid valve to the cold water supply is opened and a portion of the cold water from the tank is supplied to the heat exchange unit. A disadvantage of the above described system is that there are multiple internally piloted solenoid valves which require couplings between fluid conduits and valves which are potential for leaks. In addition, the solenoid valves are continuously actuated and are noisy. Such noise is undesirable during surgery. Moreover, such prior art systems are susceptible to malfunctions due to foreign particulates in the water, and the temperature control of such prior art systems are susceptible to thermal spikes and thermal overshoots.